1. Field of the Invention
This invention relates, in general, to electrical inductive apparatus and, more specifically, to a method of constructing electrical winding assembies for distribution transformers.
2. Description of the Prior Art
In electrical inductive apparatus, such as distribution transformers, it is common to bond the electrical windings together in a solid mass in order to provide sufficient strength to resist the mechanical forces exerted on the windings during normal operation and during extraordinary events, such as short circuits. Although many different types of adhesive and methods for applying the same have been utilized in the prior art, a conventional approach includes the use of a cellulosic sheet material which has a discontinuous coating of a dry, heat-reactive resinous material disposed on at least one side thereof, as shown in U.S. Pat. Nos. 3,237,136, 3,246,271 and 3,974,302, all assigned to the assignee of the present invention. The electrical insulative sheet material is interwoven with an electrical conductor during the construction of an electrical winding assembly such that at least one layer of the insulative sheet material is disposed between adjacent layers of the electrical conductor to provide adequate layer insulation therebetween. The electrical winding assembly is then heated to a predetermined temperature for a predetermined period of time such that the resin material on the insulative sheet material enters a semi-fluid state and flows. Then, the resin permanently sets to bond the layers of the insulative sheet material and the electrical winding together in a solid mass.
The recent trend in electrical distribution systems towards higher distribution voltages requires transformers which are capable of operating reliably at the higher distribution voltages. The higher distribution voltages and ratings necessarily increase the mechanical forces acting upon the various parts of the electrical winding of the transformer and, in particular, increase the forces acting upon the insulation space between the high and low voltage windings. Thus, it would be desirable to add additional amounts of adhesive in these areas to provide an electrical winding assembly having increased mechanical strength to resist the larger mechanical forces acting thereon and, especially, the forces occurring during a short circuit. In addition, voids formed within the resinous material as it is cured, that could be tolerated at lower voltages, create problems at the higher voltages by leading to unacceptable levels of corona and radio interference.
There are several known methods of applying resinous compositions to electrical members. One conventional approach, as shown in U.S. Pat. No. 3,451,934, is to dissolve the resinous composition in some readily volatile solvent or solvents for the purpose of simplifying the application of the resinous composition to the electrical apparatus. These solvents, which are generally low or medium boiling point liquids and not capable of entering into reaction with the resin, are removed from the resinous composition during curing by evaporation of the solvent. However, the resins generally employed have poor solvent release characteristics and thereby tend to trap portions of the solvent in the thicker deposits of resin as the resin cures to a solid state. The trapped solvent evaporates and forms voids within the solid resinous composition which trap air and leads to corona and radio interference in the electrical apparatus. It is also known to employ reacting solvents with the resinous composition in which the solvents are characterized by the fact that they enter into a polymerizing reaction with the dissolved resin. These solvent reactive resin compositions, or so-called "solventless" compositions, as disclosed in U.S. Pat. Nos. 2,484,215, 2,464,568 and 3,182,383, employ no evaporable solvent which thereby reduces the void problem associated with solvent based resinous compositions. However, these compositions are typically quite expensive for application in the electrical apparatus.
Solvent-free resinous compositions, as shown in U.S. Pat. No. 3,867,758, in which a resin is held in a water or air suspension, are also known. However, the use of a water-based composition lacks good consistency for ease of application and, further, wets and warps the cellulosic sheet material used in electrical transformers, thereby degrading the electrical characteristics of the transformer.
Thus, it is desirable to provide an electrical winding assembly for an electrical inductive apparatus, such as a distribution transformer, which has increased mechanical strength to withstand the mechanical forces acting upon the windings during extraordinary events, such as short circuits. It is also desirable to provide an electrical winding assembly wherein increased mechanical strength is obtained by the application of additional amounts of adhesive to predetermined areas within the electrical winding assembly. Finally, it is desirable to provide an electrical winding assembly which has reduced levels of corona and radio interference over similar apparatus constructed according to the prior art.